1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, the present invention relates to a semiconductor film, a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a driving method thereof, or a manufacturing method thereof, for example.
Note that in this specification, a semiconductor device refers to any device that can function by utilizing semiconductor characteristics. An electro-optical device, a display device, a memory device, a semiconductor circuit, an electronic appliance, and the like may be included in or may include a semiconductor device.
2. Description of the Related Art
Attention has been focused on a technique for forming a transistor using a semiconductor film which is formed on a substrate having an insulating surface. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. A silicon film is known as a semiconductor film applicable to a transistor.
As the silicon film used as a semiconductor film of a transistor, either an amorphous silicon film or a polycrystalline silicon film is used depending on the purpose. For example, in the case of a transistor included in a large-sized display device, it is preferred to use an amorphous silicon film, which can be formed using the established technique for forming a film on a large-sized substrate. On the other hand, in the case of a transistor included in a high-performance display device where driver circuits are formed over the same substrate, it is preferred to use a polycrystalline silicon film, which can form a transistor having a high field-effect mobility. As a method for forming a polycrystalline silicon film, high-temperature heat treatment or laser light treatment which is performed on an amorphous silicon film has been known.
In recent years, an oxide semiconductor film has attracted attention. For example, a transistor including an amorphous In—Ga—Zn oxide film is disclosed (see Patent Document 1). An oxide semiconductor film can be formed by a sputtering method or the like, and thus can be used for a semiconductor film of a transistor in a large display device. Moreover, a transistor including an oxide semiconductor film has a high field-effect mobility; therefore, a high-performance display device where driver circuits are formed over the same substrate can be obtained. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including an amorphous silicon film can be retrofitted and utilized.
Synthesis of a single crystal In—Ga—Zn oxide was reported in 1985 (see Non-Patent Document 1). Furthermore, an In—Ga—Zn oxide having a homologous structure, which is represented by a composition formula, InGaO3(ZnO)m, (m is a natural number), was reported (see Non-Patent Document 2).
In addition, a transistor including a crystalline In—Ga—Zn oxide film which has excellent electrical characteristics and reliability as compared to a transistor including an amorphous In—Ga—Zn oxide film was reported (see Non-Patent Document 3). Non-Patent Document 3 reports that a crystal boundary is not clearly observed in an In—Ga—Zn oxide film including a c-axis aligned crystal (CAAC).